The subject matter disclosed herein relates to improvements in valve and valve technology and, in particular, to improvements in balanced control valves.
Control valves include devices that regulate transmission and distribution of a working fluid (e.g., liquids and gases). These devices integrate into process control systems in a wide variety of industries, e.g., oil and gas processing, power generating, refining, petrochemical, and water control industries. These process control systems may form a control loop with remote sensors and other feedback elements to monitor process conditions (e.g., temperature, pressure, etc.). The control loop can generate signals that cause the control valve to modulate flow of the working fluid, e.g., in response to changes in the process conditions.
Examples of control valves may include a cage, a plug, a stem, and a seat ring. The valves may also have an inlet port and an outlet port. During operation, the stem can affect force onto the plug. This force can change the position of the plug relative to the seat ring to modulate flow of the working fluid between the inlet port and the outlet port. Some types of control valves (also, “balanced control valves”) allow working fluid to flow through the plug, e.g., via axial openings in the plug, to balance the pressure across the plug. These types of valves may include a seal that prevents the working fluid from leaking between the plug and the cage when the valve is closed.
Construction of the valve and, in particular, the seal may need to comport with operating conditions consistent with certain applications, processes, and/or industries. Some applications require the valve to handle working fluids at very low (e.g., cryogenic applications) or very high temperatures (e.g., temperatures of 600° C. or greater). Moreover, these applications may also require the valve to meet certain standards, e.g., standards that define minimum and maximum operating characteristics. These standards may, for example, categorize shut-off requirements for the valve as an amount of fluid that can leak downstream when the valve is closed. In one example, IEC 60534-4 defines this amount for one category of valves at a maximum of 0.0005 ml of water per minute, per inch of port diameter, per PSI differential pressure as measured from an inlet port of the valve to an outlet port of the valve. Valves in this category are often referred to as “Class V” valves.
Unfortunately, many materials and/or manufacturing techniques for components of the valves (including the seal) do not result in robust valves that can meet both the high temperature and tight shut-off requirements. For example, seals made of TEFLON® provide excellent shut-off, but are not suitable for use with working fluid at high temperatures. Seals made of graphite and metal, on the other hand, can withstand higher temperatures of the working fluid. However, components made of these materials often generate friction that may not afford the valve with adequate control performance that meets the requirements for applications that require “Class V” valves.